Created attachment 469126 [details] build.log The vala compiler stops with error message: 'error: Package `goa-1.0' not found in specified Vala API directories or GObject-Introspection GIR directories'
Created attachment 469128 [details] emerge --info
Have you compiled gnome-online-accounts without USE=introspection by any chance?
After inspection of gnome-online-accounts ebuild, it seems it doesn't not control introspection and vala dependencies properly.
This is fixed in libgdata-0.17.9-r1 and gnome-control-center 3.22.6-r1 and 3.24.2-r1. Thanks for reporting.